Document required context for `/docs` Slack requests

[kwannoel]

Context

Slack thread transcript:
[2026-02-17T22:14:27.927Z] <@U0AG7B6P9AL>: <@U0AF0DZ7C9Y> /docs

Related context search results:

• Related PRs in risingwave: 0
• Related PRs in risingwave-docs: 0
• Existing docs matches: 0
• Code matches: 0

Problem

The request only contains a bare /docs command with no problem statement, feature name, or linkable context. This makes it hard to produce a useful documentation issue and slows triage.

Suggested Fix

1. Add a short "How to request docs updates" section to contributor-facing docs describing the minimum details required in a Slack thread (what changed, where users are blocked, expected docs location, and links).
2. Provide a lightweight example template for Slack requests and example good/bad requests.
3. Ensure the docs-agent response for under-specified requests asks for missing fields in a structured checklist before filing an issue.